1. Field of the Invention
The present invention relates to a semiconductor strain sensor for detecting acceleration, strain or the like.
2. Description of the Related Art
A semiconductor acceleration sensor has been used as an automotive acceleration sensor. The sensor has a piezo-resistive element. Such a sensor is disclosed in U.S. Pat. No. 4,967,597 or Japanese patent application laid-open No. Hei2-231571. The sensor includes a cantilever and a base connecting with, and supporting the cantilever. The cantilever is composed of a silicon chip. The cantilever includes a thin flexible beam which is deformed in response to the acceleration to be measured. A piezo-resistive layer is formed in the flexible beam. The thickness of the flexible beam is generally about 40 .mu.m.
This sensor is particularly useful in an air-bag system in order to detect the variation in acceleration within a predetermined period of time. Generally, the acceleration in the air-bag system is relatively high, and ranges between 5G and 49G.
In recent years, use of the acceleration sensor in an automotive ABS (Antiskid Brake System) has been investigated. Generally, an acceleration in the ABS is relatively low, and is less than 1.5G. The sensor for predetermined time, not the variation within the predetermined period. Therefore, the output of the sensor needs to respond in a linear manner to the acceleration. Moreover, the output characteristics of the sensor preferably are not affected by variations in ambient temperature or aging. The sensor uses at least one flexible beam to strain a piezoelectric element. The thickness of this flexible beam is less than approximately 15 .mu.m to render the sensitivity of the sensor sufficiently high to enable the sensor to detect low acceleration of less than 1.5G.
Bonding strain is generated at a bonding portion between the cantilever and the base due to variations in ambient temperature or aging. The bonding strain in the acceleration sensor is caused by the difference of the thermal expansion coefficients of the cantilever and the base, while cooling after they are bonded to each other at a relatively high temperature. The bonding strain reaches the piezo-resistive layer. Therefore, when the sensor is highly sensitive, the output characteristic of the sensor are affected by the bonding strain, and the sensor does not detect low accelerations accurately.
Several techniques have been attempted to address the foregoing concern and to minimize the effect of the bonding strain on the sensitivity of the sensor. One technique has been to use the base which has a thermal expansion coefficient close to that of the cantilever. Another technique has been to bond the base and the cantilever by an anodic bonding method which tends to minimize the bonding strain. However, such conventional techniques have not been entirely satisfactory. Consequently, the highly-sensitive semiconductor acceleration sensor has not been practically used in the ABS, where only mechanical acceleration sensors have been used.